HC3: Cancer biology

Hallmarks

There are 10 hallmarks of cancer which distinguish epithelial cells from carcinomas:

1. Deregulating cellular energetics
2. Sustaining proliferative signaling
3. Evading growth suppressors
4. Avoiding immune destruction
5. Enabling replicative immortality
6. Activating invasion and metastasis
7. Inducing angiogenesis
8. Resisting cell death
9. Genome instability and mutation
10. Tumor promoting inflammation

Tumor promoting inflammation and genome instability and mutation are enabling characteristics → enhance the cancer development progress. The other hallmarks are fundamental changes in cell physiology.

Sustaining proliferative signaling

When proliferative signaling is sustained, growth is stimulated constantly:

1. A growth factor binds to a growth factor receptor
2. The growth factor receptor activates molecules, for instance Ras
3. Active Ras phosphorylates kinases
4. Kinases cause cell cycle progression
5. Cell growth

The phenotype is dominant → only 1 allele needs to be mutated. Some tumor cells:

• Secrete their own growth factors
  • Become independent of growth factors from the outside
• Modify their cell surface receptors
  • The receptors are constantly activated → don’t need growth factors anymore
• Mutate their intracellular signal molecules
• Mutate transcription factors
• Mutate components of the cell cycle network

Evading growth suppressors

Normal body cells are in equilibrium with growth suppressors and growth promotors. There are several checkpoints for controlling this:

• R-point
• DNA-integrity checkpoint
• Wnt signaling

R-point:

The R-point is the restriction point. Here, the Rb protein plays an important part → phosphorylation of Rb is necessary to release the restriction point. The Rb pathway is mutated in virtually all types of tumors. The Rb pathway can be inhibited by itself or by other means such as:

• Loss-of-function mutations of growth inhibitors
  • TGF-b
  • INK4a
• Gain-of-function mutations of growth factors
  • Cyclin D
  • CDK4

Both mutations cause Rb to become hypophosphorylated.

DNA-integrity checkpoints:

DNA-integrity checkpoints monitor 3 things:

• Whether the DNA is not too damaged
• Whether there are not mutations
• Whether the chromosome is properly attached to the spindle

A key player in this is p53, which is also called the guardian of the genome. It can be activated by:

• DNA-damage
• Hyperproliferative signals
• Telomere shortening
• Hypoxia

Activation of p53 leads to:

• Cell cycle arrest
• Senescence → not being able to replicate anymore
• Apoptosis

In more than 50% of all human tumors, p53 is mutated. p53 protective pathways are affected in more than 90% of all tumors. The following happens:

1. Loss of p53
2. Loss of DNA-integrity checkpoints → reduced apoptosis and senescence
3. Proliferation of cells with DNA damage
4. Mutations and chromosomal aberrations → genomic instability

The Li Fraumeni syndrome is a hereditary mutation. It is a heterozygous mutation of p53 which leads to multiple primary tumors at young age. It is inherited dominantly. During development, in many tissues the second allele is lost → everyone gets cancer. It is recessive on cell level.

Wnt signaling:

In case of Wnt signaling, b-catenin is degraded by antigen presenting cells (APC):

1. Wnt stimulation
2. APC releases b-catenin
3. b-catenin stabilizes and drives cell proliferation

In tumors, b-catenin is already stabilized and drives cell proliferation. Individuals with a germline APC mutation develop adenomatous polyposis coli cancer. This is a dominant cancer phenotype, which is recessive on the cell. The cancer phenotype is very dominant → at 40 years of age, the prevalence of cancer is 100%.

Avoiding immune destruction

Individuals with congenital immune deficiencies develop cancers at 200x the rate as immune-competent individuals. This rate is also increased in case of immune suppression.

There are different ways tumor antigens can be recognized:

• Normal host cell displaying multiple MHC-associated self-antigens
  • MHC shows a product of an oncogene/TSG
    • Tumor antigens
• Tumor cells expressing different types of tumor antigens
  • MHC shows a mutated self-protein
  • MHC shows overexpression of a self-protein
  • MHC shows a viral protein → viral specific T-cells react

Tumor cells can evade this by:

• Losing expression of the gene
• Losing expression of the MHC gene
• Starting expression of cytokines which suppress the recognition by T-cells

Enabling replicative immortality

Telomeres:

Normal cells have a limited proliferative capacity until they enter a state of senescence. Here, they become metabolically active and lose the ability to re-enter the cell cycle. The number of doublings depends on factors such as age and species.

This is caused by telomere shortening. A telomere is a repeating of the TTAGGG 2000 end of a chromosome. The enzyme telomerase is necessary to replicate telomeres. Because somatic cells do not express this enzyme, the chromosome becomes shorter and shorter and eventually will stop dividing.

Telomerase:

Tumor cells can re-express telomerase. There is a bridge-fusion breakage cycle → chromosomes of broken cells start sticking together → some tumor cells activate telomerase again. This makes it possible for tumor cells to endlessly divide. Tumor cells that don’t activate telomerase end up in mitotic catastrophy and die.

Tumor promoting inflammation

Tumor cells are not the only cells which are present in a tumor. There also are stromal cells and inflammatory cells. Cancer enables effects of inflammatory cells and resident stromal cells via:

• Release of factors that promote proliferation
• Removal of growth suppressors
• Enhanced resistance to cell death
• Inducing angiogenesis
• Activating invasion and metastasis
• Evading immune destruction

Activating invasion and metastasis

To metastasize, a tumor needs to:

• Loosen up tumor cell to tumor cell interaction
  • This can be done by inactivation of E-cadherin
• Degrade ECM expression
  • This can be done by expression of proteolytic enzymes
• Undergo epithelial-mesenchymal transition (EMT-transition)

Metastasizing is a very inefficient process → per day, millions of tumor cells can be released from a primary tumor of which only a few are able to metastasize.

Inducing angiogenesis

Tumors require nutrients and oxygens and need to be able to get rid of waste products → blood vessels are needed. Without blood vessels, their size is limited to 1-2 mm. Tumors grow by inducing angiogenesis. The ability to induce angiogenesis is held in balance by:

• Angiogenesis promoting proteins
• Angiogenesis inhibiting proteins

If the production of promotors is more than that of inhibitors, the tumor cells grow and develop.

Resisting cell death

The p53 response goes as follows:

1. Cells are exposed to stress → DNA damage
2. DNA damage causes activation of p53
3. p53 activates pro-apoptotic proteins
4. The mitochondrial outer membrane becomes permeable
5. Cytochrome c and APAF-1 activate caspase 9
   • APAF-1: apoptotic peptidase activating factor 1
6. Caspase 9 activates caspase 3
7. Caspase 3 leads to death substrate and apoptosis

If tumors resist cell death, the p53 response doesn’t take place. This can be caused by:

• No upregulation of pro-apoptotic factors
• Overexpression of anti-apoptotic factors
• Mutation of APAF-1
  • Caspase 9 cannot be activated
• Increase of inhibitors of apoptosis (IAP)

Reprogramming energy metabolism

Normally, humans get energy through glucose:

1. Glucose undergoes glycolysis → pyruvate
2. Pyruvate undergoes oxidative phosphorylation → 36 ATP

In proliferative tissue, only 4 mol of ATP is present because building blocks and lactate are also made. Building blocks are important for tissues which need to proliferate. This second pathway is often used in tumor cells → the Warburg effect.

Important to note is that tumor suppressing genes (TSG) suppress the Warburg effect and that lactase stimulates angiogenesis.

This can be colored in a PET or CT-scan with FDG, a glucose analog. This is a way of visualizing a tumor.

Genome instability and mutation

Genome instability and mutation is an enabling characteristic → enhances cancer development.